Are you the patent holder of this insulin, or are you talking about others simply copying the formula?
Even still, insulin manufacturing is a very complex process. It takes well over two months just to grow and make the API (active pharmaceutical ingredient.) It involves lots of customized lab equipment. You then have to aseptically make that into a solution and fill it into vials or pens, however you plan to deliver this to patients.
I would also remind you that insulin should typically be kept under refrigeration, in order to preserve its stability. That adds to the cost or both storage and distribution.
How much do you think you could sell a 10mL vial for, with all of those considerations? Could you be competitive and still make a profit?
The original patent on insulin is expired. The current patents are on “improved” versions which is why the price is high - they are like any other new drug.